Mikuni BN carb on MX engine.

BUCHAY said:
I run a44mm super bn on my Honda 450 and is everything I could ask for. My opinion, and it's just that, My opinion seems to be the opposite from everybody else, I prefer the super bn carb over the slide carbs.. in my experience, the butterfly carb is more controllable, and smoother. I have run the super bn for a lot of years and I know them so that's why I went that route, no other reason then that. The BN carb is so tunable it's not even funny. But like I said, it's my opinion and nothing more then just that.
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HI there. I'm running a sbn 38 on a gx 440. Can you help with a good pop off pressure and jet sizes to start with. And is the 1.5 needle and seat big enough for that engine thanx chris
 
If you chose a bn, you need to go bigger than you would a slide carb, the bn won't flow near as well at the same given size. A fat throttle shaft, high side fuel dump and cross bracing all cut back on airflow. that's why that 44's not too much for a 450. For engines like the 14.5 open ohv or flatty that need around 100 cfm of carb flow I'm still building Tillotson's, this is a class that has a lot of the field running slide carbs, Mikuni's mostly. The Mikuni's, which are know to have better throttle control ,seem to have no advantage for this class. So many things go into winning a race, but the carb is no doubt one key factor .I know for a fact that a Tillotson was on the winner of the Masters of the Modified's this year as well as the the $ 15,000 win at Paradise. It just goes to show the old pumper carb is not dead yet.
 
I run a SbN38 on my yz144... no issues or problems tuning it... running premium pump gas...
choke removed...
I have ran the stock slide carb that comes stock on the yz125...
but prefer the BN carb for ease of tuning...

Lap times did not change between the 2 carbs....
 
BUCHAY said:
I run a44mm super bn on my Honda 450 and is everything I could ask for. My opinion, and it's just that, My opinion seems to be the opposite from everybody else, I prefer the super bn carb over the slide carbs.. in my experience, the butterfly carb is more controllable, and smoother. I have run the super bn for a lot of years and I know them so that's why I went that route, no other reason then that. The BN carb is so tunable it's not even funny. But like I said, it's my opinion and nothing more then just that.
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HI there I wonder if you could help with jet sizes and pop off pressure on a bn 38 carb. I'm fitting it on gx 440 race motor. Needle size is a 1.5.thanxs
 
Have you ran it and now have problems ?
A 3 cylinder 1200 cc jet ski is 400cc per cyinder .
I would think its pretty close for a gx 440 .
20 psi pop off to me is high .
 
Method racing said:
HI there I wonder if you could help with jet sizes and pop off pressure on a bn 38 carb. I'm fitting it on gx 440 race motor. Needle size is a 1.5.thanxs
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Really not enough info.
Gas, or alky makes big difference.
4 stroke affects carb differently than 2 stroke because of the reeds, pumping into crankcase first.

These guys have experience with sbn carbs on larger honda engines.

http://www.vkemotorsports.com/catalog/2010_VKE_web_catalog.pdf
Save a lot of headaches, give them a call.
 
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flattop1 said:
Have you ran it and now have problems ?
A 3 cylinder 1200 cc jet ski is 400cc per cyinder .
I would think its pretty close for a gx 440 .
20 psi pop off to me is high .
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Thanxs for the reply it idles good Rev up up with a little splatter en then won't rev past 6800rpm.running a 1.5 needle and on 109 race feul. Getting 50 hp on dyno but isn't running smooth threw the Rev. And choke off at top Rev. What pop of do you think I must try and should I gou bigger with the needle valve. Problem is I'm in south africa no body run the here. Bullfrog racing run but they don't respond back to me.
 
This is all conjecture on my part . So simply consider that .
Pop off drop it to 15 , if that makes it better or worse. Adjust from there .
At wide open throttle under load , back off the throttle just a little , if it picks up its lean . Same in mide range spray a little fuel in the throat or choke it off some , again if lean it should pick up .
When you say needle are you saying the fuel inlet needle ?
I saw in the links that they make a 2.0 so that could be a concern .
The comment about 2 stroke engines needing twice the fuel holds water as it fires on every revolution .
Are you using a secondary fuel pump .
 
flattop1 said:
This is all conjecture on my part . So simply consider that .
Pop off drop it to 15 , if that makes it better or worse. Adjust from there .
At wide open throttle under load , back off the throttle just a little , if it picks up its lean . Same in mide range spray a little fuel in the throat or choke it off some , again if lean it should pick up .
When you say needle are you saying the fuel inlet needle ?
I saw in the links that they make a 2.0 so that could be a concern .
The comment about 2 stroke engines needing twice the fuel holds water as it fires on every revolution .
Are you using a secondary fuel pump .
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Pop off was at 30. My needle valve in the carb is a 1.2. I think it where my main problem is. It's not getting feul with small needle. Going to fit std jet and change needle to 1.5 and a 2 and drop the pop off n little bit.
 
No not yet it's a new motor that I trying. Had 2 different coils and flywheels. But got flat slide carb that I can try might be a good plan to try that. Where do you guy race.
 
Here is my take.

The sbn is usually ran on multi-cylinder/multi carb engines with a single carb having the pump capabilities. So if you are using a carb with the built in pump, should have capacity to supply plenty of fuel by itself. An extra pump may even be a flow restriction.

A 2 stroke will generally produce a pretty smooth one way flow, unlike a 4 stroke, which can produce reversing flow thru parts of the power curve, caused by the pipe and the valve overlap. Some carbs still meter fuel, regardless of airflow direction, so may change amount of fuel per air volume based on how much reversion you are getting.
A 2 stroke also draws air/fuel on every revolution at , usually about twice the revolutions per minute. So, times 4 may be a better comparison.

I would start, as stated above by dropping popoff to the 15-20 range. then use a tuning guide to get as good as possible.
Does your dyno have a/f testing capabilities?
 
95 shaw said:
Here is my take.

The sbn is usually ran on multi-cylinder/multi carb engines with a single carb having the pump capabilities. So if you are using a carb with the built in pump, should have capacity to supply plenty of fuel by itself. An extra pump may even be a flow restriction.

A 2 stroke will generally produce a pretty smooth one way flow, unlike a 4 stroke, which can produce reversing flow thru parts of the power curve, caused by the pipe and the valve overlap. Some carbs still meter fuel, regardless of airflow direction, so may change amount of fuel per air volume based on how much reversion you are getting.
A 2 stroke also draws air/fuel on every revolution at , usually about twice the revolutions per minute. So, times 4 may be a better comparison.

I would start, as stated above by dropping popoff to the 15-20 range. then use a tuning guide to get as good as possible.
Does your dyno have a/f testing capabilities?
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HI I'm there I'm using the carb with out the vacuum pump on it. Using a mikuni pump that also work with vacuum. I have a ect sensor and gauge I'm going to hook up know. Might be good to get a Lamda sensor for testing.
 
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